For network survivability, a protection path, in addition to a working path, is usually reserved for traffic transmission between two ends of a communication tunnel, as shown in FIG. 1. In case of a breakdown of the working path, both ends (denoted as nodes A and B in FIG. 1) switch the traffic from the working path to the protection path, so that the traffic transmission between nodes A and B can keep going.
In such a deployment as shown in FIG. 1, a certain mechanism is required to coordinate protection states between two ends of the tunnel. To fulfill this requirement in Multi-Protocol Label Switching Transport Profile (MPLS-TP) networks, a solution based on PSC state machine is given in [1]. According to that solution, for 1:1 linear protection of a MPLS-TP tunnel, the PSC state machine operating on one end of the tunnel will ignore a remote No Request (NR) message from the other end of the tunnel if the local PSC state is Do-not-Revert (DNR), and will ignore a remote DNR message if the local PSC state is Normal. Traffic from the end in a Normal state will be transmitted on the working path, whereas traffic from the end in a DNR state will be transmitted on the protection path.
In some scenarios, however, even if the PSC state machine based solution is adopted, inconsistency of PSC states between two ends of a communication tunnel cannot be avoided or eliminated. Considering for example a scenario where both the working path and the protection path went down but revive simultaneously, due to independent negotiations for recovery of the working and protection paths, there will be a possibility that node A has the working path recovered first whereas node B has the protection path recovered first, and vice versa. As a result, if the PSC domain consisting of both ends and both paths is configured by the operator not to automatically revert to the Normal state, node A will enter the Normal state and transmits traffic on the working path whereas node B will enter the DNR state and transmits traffic on the protection path, and vice versa.